Dehydrocyclization process

ABSTRACT

A process is disclosed wherein a naphtha feed is contacted in a reaction vessel with a dehydrocyclization catalyst comprising a large-pore zeolite containing at least one Group VIII metal to produce an aromatics product and a gaseous stream, the aromatics product is separated from the gaseous stream and is passed through a molecular sieve which adsorbs paraffins present in the aromatics product, then the gaseous stream is used to strip the paraffins from the molecular sieve, and the gaseous stream and the paraffins are recycled to the reaction vessel. Preferably, the dehydrocyclization catalyst comprises a type L zeolite containing from 8% to 15% by weight barium and from 0.6% to 1.0% by weight platinum, wherein at least 80% of the crystals of the type L zeolite are larger than 1000 Angstroms, and an inorganic binder selected from the group consisting of silica, alumina, and aluminosilicates.

BACKGROUND OF THE INVENTION

The invention relates to a new hydrocarbon conversion process wherein ahydrocarbon feed is contacted with a dehydrocyclization catalyst whichhas a superior selectivity for dehydrocyclization; then the paraffins inthe product stream are extracted and recycled to the reaction vessel.

Catalytic reforming is well known in the petroleum industry and refersto the treatment of naphtha fractions to improve the octane rating bythe production of aromatics. The more important hydrocarbon reactionsoccurring during reforming operation include dehydrogenation ofcyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes toaromatics, and dehydrocyclization of acyclic hydrocarbons to aromatics.A number of other reactions also occur, including the following:dealkylation of alkylbenzenes, isomerization of paraffins, andhydrocracking reactions which produce light gaseous hydrocarbons, e.g.,methane, ethane, propane and butane. Hydrocracking reactions are to beparticularly minimized during reforming as they decrease the yield ofgasoline boiling products and hydrogen.

Because of the demand for high octane gasoline for use as motor fuels,etc., extensive research is being devoted to the development of improvedreforming catalysts and catalytic reforming processes. Catalysts forsuccessful reforming processes must possess good selectivity, i.e., beable to produce high yields of liquid products in the gasoline boilingrange containing large concentrations of high octane number aromatichydrocarbons and accordingly, low yields of light gaseous hydrocarbons.The catalysts should possess good activity in order that the temperaturerequired to produce a certain quality product need not be too high. Itis also necessary that catalysts either possess good stability in orderthat the activity and selectivity characteristics can be retained duringprolonged periods of operation, or be sufficiently regenerable to allowfrequent regeneration without loss of performance.

Catalysts comprising platinum, for example, platinum supported onalumina, are well known and widely used for reforming of naphthas. Themost important products of catalytic reforming are benzene andalkylbenzenes. These aromatic hydrocarbons are of great value as highoctane number components of gasoline.

Catalytic reforming is also an important process for the chemicalindustry because of the great and expanding demand for aromatichydrocarbons for use in the manufacture of various chemical productssuch as synthetic fibers, insecticides, adhesives, detergents, plastics,synthetic rubbers, pharmaceutical products, high octane gasoline,perfumes, drying oils, ion-exchange resins, and various other productswell known to those skilled in the art. One example of this demand is inthe manufacture of alkylated aromatics such as ethylbenzene, cumene anddodecylbenzene by using the appropriate mono-olefins to alkylatebenzene. Another example of this demand is in the area of chlorinationof benzene to give chlorobenzene which is then used to prepare phenol byhydrolysis with sodium hydroxide. The chief use for phenol is in themanufacture of phenol-formaldehyde resins and plastics. Another route tophenol uses cumene as a starting material and involves the oxidation ofcumene by air to cumene hydroperoxide which can then be decomposed tophenol and acetone by the action of an appropriate acid. The demand forethylbenzene is primarily derived from its use to manufacture styrene byselective dehydrogenation; styrene is in turn used to makestyrene-butadiene rubber and polystyrene. Ortho-xylene is typicallyoxidized to phthalic anhydride by reaction in vapor phase with air inthe presence of a vanadium pentoxide catalyst. Phthalic anhydride is inturn used for production of plasticizers, polyesters and resins. Thedemand for para-xylene is caused primarily by its use in the manufactureof terephthalic acid or dimethylterephthalate which in turn is reactedwith ethylene glycol and polymerized to yield polyester fibers.Substantial demand for benzene also is associated with its use toproduce aniline, nylon, maleic anhydride, solvents and the likepetrochemical products. Toluene, on the other hand, is not, at leastrelative to benzene and the C₈ aromatics, in great demand in thepetrochemical industry as a basic building block chemical; consequently,substantial quantities of toluene are hydrodealkylated to benzene ordisproportionated to benzene and xylene. Another use for toluene isassociated with the transalkylation of trimethylbenzene with toluene toyield xylene.

Responsive to this demand for these aromatic products, the art hasdeveloped and industry has utilized a number of alternative methods toproduce them in commercial quantities. One response has been theconstruction of a significant number of catalytic reformers dedicated tothe production of aromatic hydrocarbons for use as feedstocks for theproduction of chemicals. As is the case with most catalytic processes,the principal measure of effectiveness for catalytic reforming involvesthe ability of the process to convert the feedstocks to the desiredproducts over extended periods of time with minimum interference of sidereactions.

The dehydrogenation of cyclohexane and alkylcyclohexanes to benzene andalkylbenzenes is the most thermodynamically favorable type ofaromatization reaction of catalytic reforming. This means thatdehydrogenation of cyclohexanes can yield a higher ratio of (aromaticproduct/nonaromatic reactant) than either dehydroisomerization ordehydrocyclization at a given reaction temperature and pressure.Moreover, the dehydrogenation of cyclohexanes is the fastest of thethree aromatization reactions. As a consequence of these thermodynamicand kinetic considerations, the selectivity for the dehydrogenation ofcyclohexanes is higher than that for dehydroisomerization ordehydrocyclization. Dehydroisomerization of alkylcyclopentanes issomewhat less favored, both thermodynamically and kinetically. Itsselectivity, although generally high, is lower than that fordehydrogenation. Dehydrocyclization of paraffins is much less favoredboth thermodynamically and kinetically. In conventional reforming, itsselectivity is much lower than that for the other two aromatizationreactions.

The selectivity disadvantage of paraffin dehydrocyclization isparticularly large for the aromatization of compounds having a smallnumber of carbon atoms per molecule. Dehydrocyclization selectivity inconventional reforming is very low for C₆ hydrocarbons. It increaseswith the number of carbon atoms per molecule, but remains substantiallylower than the aromatization selectivity for dehydrogenation ordehydroisomerization of naphthenes having the same number of carbonatoms per molecule. A major improvement in the catalytic reformingprocess will require, above all else, a drastic improvement indehydrocyclization selectivity that can be achieved while maintainingadequate catalyst activity and stability.

In the dehydrocyclization reaction, acyclic hydrocarbons are bothcyclized and dehydrogenated to produce aromatics. The conventionalmethods of performing these dehydrocyclization reactions are based onthe use of catalysts comprising a noble metal on a carrier. Knowncatalysts of this kind are based on alumina carrying 0.2% to 0.8% byweight of platinum and preferably a second auxiliary metal.

A disadvantage of conventional naphtha reforming catalysts is that withC₆ -C₈ paraffins, they are usually more selective for other reactions(such as hydrocracking) than they are for dehydrocyclization. A majoradvantage of the catalyst used in the present invention is its highselectivity for dehydrocyclization.

The possibility of using carriers other than alumina has also beenstudied and it was proposed to use certain molecular sieves such as Xand Y zeolites, which have pores large enough for hydrocarbons in thegasoline boiling range to pass through. However, catalysts based uponthese molecular sieves have not been commercially successful.

In the conventional method of carrying out the aforementioneddehydrocyclization, acyclic hydrocarbons to be converted are passed overthe catalyst, in the presence of hydrogen, at temperatures of the orderof 500° C. and pressures of from 5 to 30 bars. Part of the hydrocarbonsare converted into aromatic hydrocarbons, and the reaction isaccompanied by isomerization and cracking reactions which also convertthe paraffins into isoparaffins and lighter hydrocarbons.

The rate of conversion of the acyclic hydrocarbons into aromatichydrocarbons varies with the number of carbon atoms per reactantmolecule, reaction conditions and the nature of the catalyst.

The catalysts hitherto used have given moderately satisfactory resultswith heavy paraffins, but less satisfactory results with C₆ -C₈paraffins, particularly C₆ paraffins. Catalysts based on a type Lzeolite are more selective with regard to the dehydrocyclizationreaction; can be used to improve the rate of conversion to aromatichydrocarbons without requiring substantially higher temperatures thanthose dictated by thermodynamic considerations (higher temperaturesusually have a considerable adverse effect on the stability of thecatalyst); and produce excellent results with C₆ -C₈ paraffins, butcatalysts based on type L zeolite have not achieved commercial usage,apparently because of inadequate stability.

In one method of dehydrocyclizing aliphatic hydrocarbons, hydrocarbonsare contacted in the presence of hydrogen with a catalyst consistingessentially of a type L zeolite having exchangeable cations of which atleast 90% are alkali metal ions selected from the group consisting ofions of lithium, sodium, potassium, rubidium and cesium and containingat least one metal selected from the group which consists of metals ofGroup VIII of the Periodic Table of Elements, tin and germanium, saidmetal or metals including at least one metal from Group VIII of saidPeriodic Table having a dehydrogenating effect, so as to convert atleast part of the feedstock into aromatic hydrocarbons.

A particularly advantageous embodiment of this method is aplatinum/alkali metal/type L zeolite catalyst containing cesium orrubidium because of its excellent activity and selectivity forconverting hexanes and heptanes to aromatics, but stability remains aproblem.

A major recent development was the development of a newdehydrocyclization catalyst which comprises a large-pore zeolite, aGroup VIII metal, and an alkaline earth metal. This catalyst has asuperior selectivity for dehydrocyclization. This selectivity is so highthat most of the paraffins that are not dehydrocyclized remain asparaffins in the product stream, and reduce the octane rating of theresulting product.

SUMMARY OF THE INVENTION

The present invention overcomes the deficiencies of the prior art bycontacting a hydrocarbon feed with a dehydrocyclization catalystcomprising a large-pore zeolite and a Group VIII metal to produce anaromatics product and a gaseous stream; then the paraffins contained inthe aromatics product are extracted with a molecular sieve and recycledto the dehydrocyclization zone. The hydrocarbon feed is contacted with adehydrocyclization catalyst comprising a large-pore zeolite (preferablytype L zeolite), at least one Group VIII metal (preferably platinum);and preferably an alkaline earth metal selected from the groupconsisting of barium, strontium and calcium (preferably barium). Theresult of the dehydrocyclization is an aromatics product and a gaseousstream. The aromatics product is separated from the gaseous stream andis passed through a molecular sieve which adsorbs the paraffins presentin the aromatics product; then the gas stream is used to strip theparaffins from the molecular sieve and both the gaseous product and theparaffins are recycled to the dehydrocyclization zone.

Preferably, the dehydrocyclization catalyst contains: (a) a type Lzeolite containing from 0.1% to 5% by weight platinum (preferably from0.6% to 1.0% by weight platinum) and 0.1% to 40% by weight barium(preferably from 0.1% to 35% by weight barium, more preferably from 8%to 15% by weight barium); and (b) an inorganic binder. The majority ofthe type L zeolite crystals are preferably greater than 500 Angstoms,more preferably greater than 1000 Angstroms. In the most preferredembodiment, at least 80% of the crystals of type L zeolite are greaterthan 1000 Angstoms. The inorganic binder is preferably either a silica,alumina, or an aluminosilicate. The hydrocarbons are contacted with thebarium-exchanged type zeolite at a temperature of from 400° C. to 600°C. (preferably 450° C. to 550° C.); an LHSV of from 0.1 to 20(preferably from 0.3 to 5); a pressure of from 1 atmosphere to 500 psig(preferably from 50 to 200 psig); and an H₂ /HC ratio of from 0 to 20:1(preferably from 2:1 to 6:1).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In its broadest aspect, the present invention involves the extractionand recycle of paraffins present in the product stream of adehydrocyclization process using a dehydrocyclization catalyst having avery high selectivity for dehydrocyclization. This dehydrocyclization iscarried out using a dehydrocyclization catalyst comprising a large-porezeolite and a Group VIII metal.

The term "selectivity" as used in the present invention is defined asthe percentage of moles of acyclic hydrocarbons converted to aromaticsrelative to moles converted to aromatics and cracked products, ##EQU1##

Isomerization of paraffins and interconversion of paraffins andalkylcyclopentanes having the same number of carbon atoms per moleculeare not considered in determining selectivity.

The selectivity for converting acyclic hydrocarbons to aromatics is ameasure of the efficiency of the process in converting acyclichydrocarbons to the desired and valuable products: aromatics andhydrogen, as opposed to the less desirable products of hydrocracking.

Highly selective catalysts produce more hydrogen than less selectivecatalysts because hydrogen is produced when acyclic hydrocarbons areconverted to aromatics and hydrogen is consumed when acyclichydrocarbons are converted to cracked products. Increasing theselectivity of the process increases the amount of hydrogen produced(more aromatization) and decreases the amount of hydrogen consumed (lesscracking).

Another advantage of using highly selective catalysts is that thehydrogen produced by highly selective catalysts is purer than thatproduced by less selective catalysts. This higher purity results becausemore hydrogen is produced, while less low boiling hydrocarbons (crackedproducts) are produced. The purity of hydrogen produced in reforming iscritical if, as is usually the case in an integrated refinery, thehydrogen produced is utilized in processes such as hydrotreating andhydrocracking, which require at least certain minimum partial pressuresof hydrogen. If the purity becomes too low, the hydrogen can no longerbe used for this purpose and must be used in a less valuable way, forexample as fuel gas.

Feedstock

Regarding the dehydrocyclizable hydrocarbons that are subjected to themethod of the present invention, they can in general be any aliphatichydrocarbon or substituted aliphatic hydrocarbon capable of undergoingring-closure to produce an aromatic hydrocarbon. That is, it is intendedto include within the scope of the present invention, thedehydrocyclization of any organic compound capable of undergoingring-closure to produce an aromatic hydrocarbon and capable of beingvaporized at the dehydrocyclization temperatures used herein. Moreparticularly, suitable dehydrocyclizable hydrocarbons are: aliphatichydrocarbons containing 6 to 20 carbon atoms per molecule such as C₆-C₂₀ paraffins, C₆ -C₂₀ olefins and C₆ -C₂₀ polyolefins. Specificexamples of suitable dehydrocyclizable hydrocarbons are: (1) paraffinssuch as n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane,n-heptane, 2-methylhexane, 3-ethylpentane, 2,2-dimethylpentane,n-octane, 2-methylheptane, 3-ethylhexane, 2,2-dimethylhexane,2-methyl-3-ethylpentane, 2,2,3-trimethylpentane, n-nonane,2-methyloctane, 2,2-dimethylheptane, n-decane and the like compounds;(2) olefins such as 1-hexene, 2-methyl-1-pentene, 1-heptene, 1-octene,1-nonene and the like compounds; and (3) diolefins such as1,5-hexadiene, 2-methyl-2,4-hexadiene, 2,6-octadiene and the likediolefins.

In a preferred embodiment, the dehydrocyclizable hydrocarbon is aparaffin hydrocarbon having about 6 to 12 carbon atoms per molecule. Forexample, paraffin hydrocarbons containing about 6 to 10 carbon atoms permolecule are dehydrocyclized by the subject method to produce thecorresponding aromatic hydrocarbon. It is to be understood that thespecific dehydrocyclizable hydrocarbons mentioned above can be chargedto the present method individually, in admixture with one or more of theother dehydrocyclizable hydrocarbons, or in admixture with otherhydrocarbons such as naphthenes, aromatics and the like. Thus, mixedhydrocarbon fractions, containing significant quantities ofdehydrocyclizable hydrocarbons that are commonly available in a typicalrefinery, are suitable charge stocks for the instant method; forexample, highly paraffinic straight-run naphthas, paraffinic raffinatesfrom aromatic extraction or adsorption, C₆ -C₁₂ paraffin-rich streamsand the like refinery streams. An especially preferred embodimentinvolves a charge stock which is a paraffin-rich naphtha fractionboiling in the range of about 140° F. to about 400° F. Generally, bestresults are obtained with a charge stock comprising a mixture of C₆ -C₁₂paraffins.

Preferably, the feedstock is substantially free of sulfur, nitrogen,metals, and other known poisons for reforming catalysts. This catalystis especially sensitive to sulfur. The feedstock can be madesubstantially free of sulfur, nitrogen, metals, and other known poisonsby conventional hydrofining techniques plus sorbers that remove sulfurcompounds.

In the case of a feedstock which is not already low in sulfur,acceptable levels can be reached by hydrofining the feedstock in apretreatment zone where the naphtha is contacted with a hydrofiningcatalyst which is resistant to sulfur poisoning. A suitable catalyst forthis hydrodesulfurization process is, for example, an alumina-containingsupport and a minor proportion of molybdenum oxide, cobalt oxide and/ornickel oxide. Hydrodesulfurization is ordinarily conducted at 315° C. to455° C., at 200 to 2000 psig, and at a liquid hourly space velocity of 1to 5. The sulfur and nitrogen contained in the naphtha are converted tohydrogen sulfide and ammonia, respectively, which can be removed priorto reforming by suitable conventional processes.

Dehydrocyclization Reaction

According to the present invention, the hydrocarbon feedstock iscontacted with the catalyst in a dehydrocyclization zone maintained atdehydrocyclization conditions. This contacting may be accomplished byusing the catalyst in a fixed bed system, a moving bed system, afluidized system, or in a batch-type operation; however, in view of thedanger of attrition losses of the valuable catalyst and of well-knownoperational advantages, it is preferred to use either a fixed bed systemor a densephase moving bed system. It is also contemplated that thecontacting step can be performed in the presence of a physical mixtureof particles of a conventional dual-function catalyst of the prior art.In a fixed bed system, the hydrocarbons in the C₆ to C₁₄ range arepreheated by any suitable heating means to the desired reactiontemperature and then passed into a dehydrocyclization zone containing afixed bed of the catalyst. It is, of course, understood that thedehydrocyclization zone may be one or more separate reactors withsuitable means therebetween to ensure that the desired conversiontemperature is maintained at the entrance to each reactor. It is alsoimportant to note that the reactants may be contacted with the catalystbed in either upward, downward, or radial flow fashion. In addition, thereactants may be in a liquid phase, a mixed liquid-vapor phase, or avapor phase when they contact the catalyst, with best results obtainedin the vapor phase. The dehydrocyclization system then preferablycomprises a dehydrocyclization zone containing one or more fixed beds ordense-phase moving beds of the catalyst. In a multiple bed system, itis, of course, within the scope of the present invention to use thepresent catalyst in less than all of the beds with a conventionaldual-function catalyst being used in the remainder of the beds. Thedehydrocyclization zone may be one or more separate reactors withsuitable heating means therebetween to compensate for the endothermicnature of the dehydrocyclization reaction that takes place in eachcatalyst bed.

Although hydrogen is the preferred diluent for use in the subjectdehydrocyclization method, in some cases other art-recognized diluentsmay be advantageously utilized, either individually or in admixture withhydrogen, such as C₁ to C₅ paraffins such as methane, ethane, propane,butane and pentane; the like diluents, and mixtures thereof. Hydrogen ispreferred because it serves the dual function of not only lowering thepartial pressure of the acyclic hydrocarbon, but also of suppressing theformation of hydrogen-deficient, carbonaceous deposits (commonly calledcoke) on the catalytic composite. Ordinarily, hydrogen is utilized inamounts sufficient to insure a hydrogen to hydrocarbon mole ratio ofabout 0 to about 20:1, with best results obtained in the range of about2:1 to about 6:1. The hydrogen charged to the dehydrocyclization zonewill typically be contained in a hydrogen-rich gas stream recycled fromthe effluent stream from this zone after a suitable gas/liquidseparation step.

The hydrocarbon dehydrocyclization conditions used in the present methodinclude a reactor pressure which is selected from the range of about 1atmosphere to about 500 psig, with the preferred pressure being about 50psig to about 200 psig. The temperature of the dehydrocyclization ispreferably about 450° C. to about 550° C. As is well known to thoseskilled in the dehydrocyclization art, the initial selection of thetemperature within this broad range is made primarily as a function ofthe desired conversion level of the acyclic hydrocarbon considering thecharacteristics of the charge stock and of the catalyst. Ordinarily, thetemperature then is thereafter slowly increased during the run tocompensate for the inevitable deactivation that occurs to provide arelatively constant value for conversion.

The liquid hourly space velocity (LHSV) used in the instantdehydrocyclization method is selected from the range of about 0.1 toabout 20 hr.⁻¹, with a value in the range of about 0.3 to about 5 hr.⁻¹being preferred.

Reforming generally results in the production of hydrogen. Thus,exogenous hydrogen need not necessarily be added to the reforming systemexcept for pre-reduction of the catalyst and when the feed is firstintroduced. Generally, once reforming is underway, part of the hydrogenproduced is recirculated over the catalyst. The presence of hydrogenserves to reduce the formation of coke which tends to poison thecatalyst. Hydrogen is preferably introduced into the reforming reactorat a rate varying from 0 to about 20 moles of hydrogen per mole of feed.The hydrogen can be in admixture with light gaseous hydrocarbons.

If, after a period of operation, the catalyst has become deactivated bythe presence of carbonaceous deposits, said deposits can be removed fromthe catalyst by passing an oxygen-containing gas, such as dilute air,into contact with the catalyst at an elevated temperature in order toburn the carbonaceous deposits from the catalyst. The regeneration canbe performed either in the semiregenerative mode in which the reformingoperation is interrupted after a more or less long period of time andcatalyst regeneration is carried out, or in the onstream regenerativemode, in which a portion of the catalyst is regenerated while thereforming operation is continued over the remainder of the catalyst. Twotypes of onstream regeneration are known in the prior art, cyclic andcontinuous reforming. In cyclic reforming, the catalyst in one of aseries of reactors is regenerated while reforming is continued in therest of the plant. In continuous reforming, a portion of deactivatedcatalyst is removed from the plant, regenerated in a separateregeneration system while reforming is continued in the plant, and theregenerated catalyst is returned to the plant. The method ofregenerating the catalyst will depend on whether there is a fixed bed,moving bed, or fluidized bed operation. Regeneration methods andconditions are well known in the art.

The Dehydrocyclization Catalyst

The dehydrocyclization catalyst according to the invention is alarge-pore zeolite charged with one or more dehydrogenatingconstituents. The term "large-pore zeolite" is defined as a zeolitehaving an effective pore diameter of 6 to 15 Angstroms.

Among the large-pored crystalline zeolites which have been found to beuseful in the practice of the present invention, type L zeolite, zeoliteX, zeolite Y and faujasite are the most important and have apparent poresizes on the order of 7 to 9 Angstroms.

The chemical formula for zeolite Y expressed in terms of mole oxides maybe written as:

    (0.7-1.1)Na.sub.2 O:Al.sub.2 O.sub.3 :xSiO.sub.2 :yH.sub.2 O

wherein x is a value greater than 3 up to about 6 and y may be a valueup to about 9. Zeolite Y has a characteristic X-ray powder diffractionpattern which may be employed with the above formula for identification.Zeolite Y is described in more detail in U.S. Pat. No. 3,130,007. U.S.Pat. No. 3,130,007 is hereby incorporated by reference to show a zeoliteuseful in the present invention.

Zeolite X is a synthetic crystalline zeolitic molecular sieve which maybe represented by the formula:

    (0.7-1.1)M.sub.2/n O:Al.sub.2 O.sub.3 :(2.0-3.0)SiO.sub.2 :yH.sub.2 O

wherein M represents a metal, particularly alkali and alkaline earthmetals, n is the valence of M, and y may have any value up to about 8depending on the identity of M and the degree of hydration of thecrystalline zeolite. Zeolite X, its X-ray diffraction pattern, itsproperties, and method for its preparation are described in detail inU.S. Pat. No. 2,882,244. U.S. Pat. No. 2,882,244 is hereby incorporatedby reference to show a zeolite useful in the present invention.

The preferred catalyst according to the invention is a type L zeolitecharged with one or more dehydrogenating constituents.

Type L zeolites are synthetic zeolites. A theoretical formula is M₉ /n[(AlO₂)₉ (SiO₂)₂₇ ] in which M is a cation having the valency n.

The real formula may vary without changing the crystalline structure;for example, the mole ratio of silicon to aluminum (Si/Al) may vary from1.0 to 3.5.

Although there are a number of cations that may be present in zeolite L,in one embodiment, it is preferred to synthesize the potassium form ofthe zeolite, i.e., the form in which the exchangeable cations presentare substantially all potassium ions. The reactants accordingly employedare readily available and generally water soluble. The exchangeablecations present in the zeolite may then conveniently be replaced byother exchangeable cations, as will be shown below, thereby yieldingisomorphic form of zeolite L.

In one method of making zeolite L, the potassium form of zeolite L isprepared by suitably heating an aqueous metal aluminosilicate mixturewhose composition, expressed in terms of the mole ratios of oxides,falls within the range:

    ______________________________________                                        K.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                             From about 0.33 to about 1                                   (K.sub.2 O + Na.sub.2 O)/SiO.sub.2                                                             From about 0.35 to about 0.5                                 SiO.sub.2 /Al.sub.2 O.sub.3                                                                    From about 10 to about 28                                    H.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                             From about 15 to about 41                                    ______________________________________                                    

The desired product is hereby crystallized out relatively free fromzeolites of dissimilar crystal structure.

The potassium form of zeolite L may also be prepared in another methodalong with other zeolitic compounds by employing a reaction mixturewhose composition, expressed in terms of mole ratios of oxides, fallswithin the following range:

    ______________________________________                                        K.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                             From about 0.26 to about 1                                   (K.sub.2 O + Na.sub.2 O)/SiO.sub.2                                                             From about 0.34 to about 0.5                                 SiO.sub.2 /Al.sub.2 O.sub.3                                                                    From about 15 to about 28                                    H.sub.2 O/(K.sub.2 O + Na.sub.2 O)                                                             From about 15 to about 51                                    ______________________________________                                    

It is to be noted that the presence of sodium in the reaction mixture isnot critical to the present invention.

When the zeolite is prepared from reaction mixtures containing sodium,sodium ions are generally also included within the product as part ofthe exchangeable cations together with the potassium ions. The productobtained from the above ranges has a composition, expressed in terms ofmoles of oxides, corresponding to the formula:

    0.9-1.3[(1-x)K.sub.2 O, xNa.sub.2 O]:Al.sub.2 O.sub.3 :5.2-6.9SiO.sub.2 :yH.sub.2 O

wherein "x" may be any value from 0 to about 0.75 and "y" may be anyvalue from 0 to about 9.

In making zeolite L, representative reactants are activated alumina,gamma alumina, alumina trihydrate and sodium aluminate as a source ofalumina. Silica may be obtained from sodium or potassium silicate,silica gels, silicic acid, aqueous colloidal silica sols and reactiveamorphous solid silicas. The preparation of typical silica sols whichare suitable for use in the process of the present invention aredescribed in U.S. Pat. No. 2,574,902 and U.S. Pat. No. 2,597,872.Typical of the group of reactive amorphous solid silicas, preferablyhaving an ultimate particle size of less than 1 micron, are suchmaterials as fume silicas, chemically precipitated and precipitatedsilica sols. Potassium and sodium hydroxide may supply the metal cationand assist in controlling pH.

In making zeolite L, the usual method comprises dissolving potassium orsodium aluminate and alkali, viz., potassium or sodium hydroxide, inwater. This solution is admixed with a water solution of sodiumsilicate, or preferably with a water-silicate mixture derived at leastin part from an aqueous colloidal silica sol. The resultant reactionmixture is placed in a container made, for example, of metal or glass.The container should be closed to prevent loss of water. The reactionmixture is then stirred to insure homogeneity.

The zeolite may be satisfactorily prepared at temperatures of from about90° C. to 200° C. the pressure being atmospheric or at least thatcorresponding to the vapor pressure of water in equilibrium with themixture of reactants at the higher temperature. Any suitable heatingapparatus, e.g., an oven, sand bath, oil bath or jacketed autoclave, maybe used. Heating is continued until the desired crystalline zeoliteproduct is formed. The zeolite crystals are then filtered off and washedto separate them from the reactant mother liquor. The zeolite crystalsshould be washed, preferably with distillated water, until the effluentwash water, in equilibrium with the product, has a pH of between about 9and 12. As the zeolite crystals are washed, the exchangeable cation ofthe zeolite may be partially removed and is believed to be replaced byhydrogen cations. If the washing is discontinued when the pH of theeffluent wash water is between about 10 and 11, the (K₂ O+Na₂ O)/Al₂ O₃molar ratio of the crystalline product will be approximately 1.0.Thereafter, the zeolite crystals may be dried, conveniently in a ventedoven.

Zeolite L has been characterized in "Zeolite Molecular Sieves" by DonaldW. Breck, John Wiley & Sons, 1974, as having a framework comprising 18tetrahedra unit cancrinite-type cages linked by double 6-rings incolumns and crosslinked by single oxygen bridges to form planar12-membered rings. These 12-membered rings produce wide channelsparallel to the c-axis with no stacking faults. Unlike erionite andcancrinite, the cancrinite cages are symmetrically placed across thedouble 6-ring units. There are four types of cation locations: A in thedouble 6-rings, B in the cancrinite-type cages, C between thecancrinite-type cages, and D on the channel wall. The cations in site Dappear to be the only exchangeable cations at room temperature. Duringdehydration, cations in site D probably withdraw from the channel wallsto a fifth site, site E, which is located between the A sites. Thehydrocarbon sorption pores are approximately 7 to 8 Angstroms indiameter.

A more complete description of these zeolites is given, e.g., in U.S.Pat. No. 3,216,789 which, more particularly, gives a conventionaldescription of these zeolites. U.S. Pat. No. 3,216,789 is herebyincorporated by reference to show a type L zeolite useful in the presentinvention.

Zeolite L differs from other large pore zeolites in a variety of ways,besides X-ray diffraction pattern.

One of the most pronounced differences is in the channel system ofzeolite L. Zeolite L has a one-dimensional channel system parallel tothe c-axis, while most other zeolites have either two-dimensional orthree-dimensional channel systems. Zeolite A, X and Y all havethree-dimensional channel systems. Mordenite (Large Port) has a majorchannel system parallel to the c-axis, and another very restrictedchannel system parallel to the b-axis. Omega zeolite has aone-dimensional channel system.

Another pronounced difference is in the framework of the variouszeolites. Only zeolite L has cancrinite-type cages linked by double-sixrings in columns and crosslinked by oxygen bridges to form planar12-rings. Zeolite A has a cubic array of truncated octahedra, beta-cageslinked by double-four ring units. Zeolites X and Y both have truncatedoctahedra, beta-cages, linked tetrahedrally through double-six rings inan arrangement like carbon atoms in a diamond. Mordenite has complexchains of five-rings crosslinked by four-ring 25 chains. Omega has afourteen-hedron of gmelinite-type linked by oxygen bridges in columnsparallel to the c-axis.

Presently, it is not known which of these differences, or otherdifferences, is responsible for the high selectivity fordehydrocyclization of catalysts made from zeolite L, but it is knownthat catalysts made of zeolite L do react differently than catalystsmade of other zeolites.

Various factors have an effect on the X-ray diffraction pattern of azeolite. Such factors include temperature, pressure, crystal size,impurities, and type of cations present. For instance, as the crystalsize of the type L zeolite becomes smaller, the X-ray diffractionpattern becomes broader and less precise. Thus, the term "zeolite L"includes any zeolites made up of cancrinite cages having an X-raydiffraction pattern substantially similar to the X-ray diffractionpatterns shown in U.S. Pat. No. 3,216,789.

Crystal size also has an effect on the stability of the catalyst. Forreasons not yet fully understood, catalysts having at least 80% of thecrystals of the type L zeolite larger than 1000 Angstroms give longerrun length than catalysts having substantially all of the crystals ofthe type L zeolite between 200 and 500 Angstroms. Thus, the larger ofthese crystallite sizes of type L zeolite is the preferred support.

Type L zeolites are conventionally synthesized largely in the potassiumform, i.e., in the theoretical formula given previously, most of the Mcations are potassium. The M cations are exchangeable, so that a giventype L zeolite, e.g., a type L zeolite in the potassium form, can beused to obtain type L zeolites containing other cations, by subjectingthe type L zeolite to ion exchange treatment in an aqueous solution ofappropriate salts. However, it is difficult to exchange all of theoriginal cations, e.g., potassium, since some exchangeable cations inthe zeolite are in sites which are difficult for the reagents to reach.

Alkaline Earth Metals

A preferred element of the present invention is the presence of analkaline earth metal in the dehydrocyclization catalyst. That alkalineearth metal must be either barium, strontium or calcium. Preferably thealkaline earth metal is barium. The alkaline earth metal can beincorporated into the zeolite by synthesis, impregnation or ionexchange. Barium is preferred to the other alkaline earths because theresulting catalyst has high activity, high selectivity and highstability.

In one embodiment, at least part of the alkali metal is exchanged withbarium, using techniques known for ion exchange of zeolites. Thisinvolves contacting the zeolite with a solution containing excess Ba⁺⁺ions. The barium should preferably constitute from 0.1% to 35% of theweight of the zeolite, more preferably from 8% to 15% by weight.

Group VIII Metals

The dehydrocyclization catalysts according to the invention are chargedwith one or more Group VIII metals, e.g., nickel, ruthenium, rhodium,palladium, iridium or platinum.

The preferred Group VIII metals are iridium and particularly platinum,which are more selective with regard to dehydrocyclization and are alsomore stable under the dehydrocyclization reaction conditions than otherGroup VIII metals.

The preferred percentage of platinum in the catalyst is between 0.1% and5%, more preferably from 0.6% to 1.0%.

Group VIII metals are introduced into the zeolite by synthesis,impregnation or exchange in an aqueous solution of an appropriate salt.When it is desired to introduce two Group VIII metals into the zeolite,the operation may be carried out simultaneously or sequentially.

By way of example, platinum can be introduced by impregnating thezeolite with an aqueous solution of tetrammineplatinum (II) nitrate,tetrammineplatinum (II) hydroxide, dinitrodiamino-platinum ortetrammineplatinum (II) chloride. In an ion exchange process, platinumcan be introduced by using cationic platinum complexes such astetrammineplatinum (II) nitrate.

Catalyst Pellets

An inorganic oxide can be used as a carrier to bind the zeolitecontaining the Group VIII metal and alkaline earth metal and give thedehydrocyclization catalyst additional strength. The carrier can be anatural or a synthetically produced inorganic oxide or combination ofinorganic oxides. Preferred loadings of inorganic oxide are from 5% to25% by weight of the catalyst. Typical inorganic oxide supports whichcan be used include aluminosilicates (such as clays), alumina, andsilica, in which acidic sites are preferably exchanged by cations whichdo not impart strong acidity.

One preferred inorganic oxide support is attapulgite. Another preferredinorganic oxide support is "Ludox", which is a colloidal suspension ofsilica in water, stabilized with a small amount of alkali.

When an inorganic oxide is used as a carrier, there are two preferredmethods in which the catalyst can be made, although other embodimentscould be used.

In the first preferred embodiment, the zeolite is made, then the zeoliteis ion exchanged with a barium solution, separated from the bariumsolution, dried and calcined, impregnated with platinum, calcined, andthen mixed with the inorganic oxide and extruded through a die to formcylindrical pellets. Advantageous methods of separating the zeolite fromthe barium and platinum solutions are by a batch centrifuge or a pressedfilter. This embodiment has the advantage that all the barium andplatinum are incorporated on the zeolite and none are incorporated onthe inorganic oxide. It has the disadvantage that the large-pore zeoliteis of small size, which is hard to separate from the barium solution andthe platinum solution.

In the second preferred embodiment, the large-pore zeolite is mixed withthe inorganic oxide and extruded through the die to form cylindricalpellets, then these pellets are ion exchanged with a barium solution,separated from the barium solution, impregnated with platinum, separatedfrom the platinum solution, and calcined. This embodiment has theadvantage that the pellets are easy to separate from the barium andplatinum solutions, but it has the disadvantage that barium and platinummay be also deposited on the inorganic oxide carrier which couldcatalyze undesirable reactions. Thus, the choice of which embodiment isused depends on the trade-off between catalyst selectivity and ease ofseparation of the catalyst from the barium and platinum solutions.

In the extrusion of large-pore zeolite, various extrusion aids and poreformers can be added. Examples of suitable extrusion aids are ethyleneglycol and stearic acid. Examples of suitable pore formers are woodflour, cellulose and polyethylene fibers.

After the desired Group VIII metal or metals have been introduced, thecatalyst is treated in air at about 260° C. and then reduced in hydrogenat temperatures of from 200° C. to 700° C., preferably 300° C. to 620°C.

At this stage the dehydrocyclization catalyst is ready for use in thedehydrocyclization process. In some cases however, for example when themetal or metals have been introduced by an ion exchange process, it ispreferable to eliminate any residual acidity of the zeolite by treatingthe catalyst with an aqueous solution of a salt of a suitable alkali oralkaline earth element in order to neutralize any hydrogen ions formedduring the reduction of metal ions by hydrogen.

In order to obtain optimum selectivity, temperature should be adjustedso that reaction rate is appreciable, but conversion is less than 80%,as excessive temperature and excess reaction can have an adverse affecton selectivity. Pressure should also be adjusted within a proper range.Too high a pressure will place a thermodynamic (equilibrium) limit onthe desired reaction, especially for hexane aromatization.

Since the selectivity of this dehydrocyclization catalyst is high, theproduct stream from the dehydrocyclization is comprised predominantly ofaromatics and paraffins, plus a small gaseous stream. The aromatics andparaffins are then separated from the gaseous stream using a highpressure separator or other conventional separation technology.

Most of the paraffins are extracted from the aromatics by passing thearomatics and paraffins through a molecular sieve which adsorbs thenormal paraffins and some of the isoparaffins present, but not thearomatics. To cause such a separation, the molecular sieve should havean effective pore diameter of from 4.5 to 5.5 Angstroms. Examples ofsuch molecular sieves are A, X, Y, offretite and ZSM, with cationsproperly used to tailor the size of zeolite opening to accommodate thedesired separation.

The gaseous stream is then used to strip the paraffins from themolecular sieve. Then both the gaseous stream and the paraffins strippedfrom the molecular sieve are recycled to the dehydrocyclization zone.Since both the gaseous stream and the paraffins are recycled, there isno need to separate the paraffins from the gaseous stream.

Thus, by this process, a highly selective dehydrocyclization catalystcan be used without the octane penalty resulting from the presence ofexcess paraffins. Instead, these paraffins are converted to high octanearomatics.

Another major advantage of this process is that, since the paraffins arerecycled, the severity of the dehydrocyclization reaction can be reducedand still achieve the same yields and octane numbers as when a higherseverity dehydrocyclization is used without recycle. This means that thereaction can be operated at lower temperatures, which will result inlonger run times.

While the present invention has been described with reference tospecific embodiments, this application is intended to cover thosevarious changes and substitutions which may be made by those skilled inthe art without departing from the spirit and scope of the appendedclaims.

What is claimed is:
 1. A dehydrocyclization process comprising:(a)contacting a naphtha feed in a reaction vessel with a dehydrocyclizationcatalyst at process conditions which favor dehydrocyclization to producean aromatics product and a gaseous stream, wherein saiddehydrocyclization catalyst is a monofunctional catalyst comprising alarge-pore zeolite containing at least one Group VIII metal; (b)separating said aromatics product from said gaseous stream; (c) passingsaid aromatics product through a molecular sieve which adsorbs normalparaffins and a substantial portion of the single-branched isoparaffinspresent in said aromatics product; (d) using the gaseous stream to stripsaid normal paraffins and single-branched isoparaffins from themolecular sieve; and (e) recycling said gaseous stream and said normalparaffins and single-branched isoparaffins to said reaction vessel.
 2. Adehydrocyclization process according to claim 1 wherein said molecularsieve is zeolite A.
 3. A dehydrocyclization process according to claim 1wherein said dehydrocyclization catalyst contains an alkaline earthmetal selected from the group consisting of barium, strontium, andcalcium.
 4. A dehydrocyclization process according to claim 3 whereinsaid alkaline earth metal is barium and wherein said Group VIII metal isplatinum.
 5. A dehydrocyclization process according to claim 4 whereinsaid dehydrocyclization catalyst has from 0.1% to 35% by weight bariumand from 0.1% to 5% by weight platinum.
 6. A dehydrocyclization processaccording to claim 5 wherein said dehydrocyclization catalyst has from8% to 15% by weight barium and from 0.6% to 1.0% by weight platinum. 7.A dehydrocyclization process according to claim 1 wherein saidlarge-pore zeolite has an apparent pore size of from 7 to 9 Angstroms.8. A dehydrocyclization process according to claim 7 wherein saidlarge-pore zeolite is selected from the group consisting of zeolite X,zeolite Y and type L zeolite.
 9. A dehydrocyclization process accordingto claim 8 wherein said large-pore zeolite is zeolite Y.
 10. Adehydrocyclization process according to claim 8 wherein said large-porezeolite is a type L zeolite.
 11. A dehydrocyclization process accordingto claim 10 wherein the majority of the crystals of said type L zeoliteare larger than 500 Angstroms.
 12. A dehydrocyclization processaccording to claim 11 wherein the majority of the crystals of said typeL zeolite are larger than 1000 Angstroms.
 13. A dehydrocyclizationprocess according to claim 12 wherein at least 80% of the crystals ofsaid type L zeolite are larger than 1000 Angstroms.
 14. Adehydrocyclization process accorcing to claim 1 wherein saiddehydrocyclization catalyst comprises:(a) a large-pore zeolitecontaining barium and platinum; and (b) an inorganic binder.
 15. Adehydrocyclization process according to claim 14 said inorganic binderis selected from the group consisting of silica, alumina, andaluminosilicates.
 16. A dehydrocyclization process comprising:(a)contacting a naphtha feed in a reaction vessel with a dehydrocyclizationcatalyst at process conditions which favor dehydrocyclization to producean aromatics product and a gaseous stream; (b) separating said aromaticsproduct from said gaseous stream; (c) passing said aromatics productthrough a molecular sieve which adsorbs normal paraffics andsingle-branched isoparaffins present in said aromatics product; (d)using the gaseous stream to strip said normal paraffins andsingle-branched isoparaffins from the molecular sieve; and (e) recyclingsaid gaseous stream and said normal paraffins and single-branchedisoparaffins to said reaction vessel, wherein said dehydrocyclizationcatalyst comprises:(1) a type L zeolite containing from 8% to 15% byweight barium and from 0.6% to 1.0% by weight platinum, wherein at least80% of the crystals of said type L zeolite are larger than 1000Angstroms; and (2) an inorganic binder selected rrom the groupconsisting of silica, alumina, and aluminosilicates.